Fiber bed separators have found widespread use in applications wherein extremely fine aerosols of under 3 microns, and particularly under 1 micron, in particle size must be separated from a gas or vapor (hereinafter and in the claims collectively referred to as gas) stream. Fiber beds of up to 20 micron fibers have been found to remove sub-micron up to 3 micron sized aerosols with high collection efficiency, for example, as high as 98-99.9% efficiency. In applications requiring or permitting treatment of such aerosol containing gases at high bed velocities, such as for example 300 feet per minute (91.4 meters per minute) or more, larger diameter fibers have been used, e.g., about 25 to 50 microns, with some sacrifice in collection efficiency but even then efficiencies of 85 to 95% are attainable. Some of the more frequent applications include removal of acid mists, such as sulfuric acid mist, in acid manufacturing processes, plasticizer mists in, for example, polyvinyl chloride floor or wall covering manufacture, and water soluble solid aerosols such as, for example, emissions from ammonium nitrate prill towers. In removal of water soluble solid aerosols, the collected particulates are dissolved in a liquid within the fiber bed through use of an irrigated fiber bed or of a fogging spray of liquid such as water injected into the gas stream prior to the fiber bed.
Re-entrainment of collected liquid from the down-stream surface of the fiber bed is often a problem with fiber bed separators. When the aerosol in the gas being treated is a mixture of particulates ranging in size from sub-micron to a few microns, the re-entrained particles normally have been coalesced in the fiber bed to a much larger average size. The large drops present a problem in that some supplemental removal must be performed downstream, but a much greater problem with re-entrainment is that a significant amount of sub-micron up to several micron in size particles are also formed which present a much more difficult downstream separation problem than do the larger droplet sized particles. In the past, this re-entrainment problem has been handled in a variety of ways.
A downstream impingement baffle can be used whereby the gas is caused to change its direction of flow by the baffle while re-entrained particles of heavier mass impinge on the baffle surface and drain down. This is adequate for removal of particles of great enough size and mass that their inertia will cause them to strike the baffle rather than continue to flow with the gas around the baffle. It does not effectively remove, however, the small particles below about 3 microns in size, which, because of their low mass, will tend to flow around the baffle and continue with the gas stream.
Argo et al. U.S. Pat. No. 4,086,070 describes a fiber bed separator containing a composite fiber bed element in which the first bed is comprised of relatively fine fibers and the second bed of relatively coarse fibers. Collection of the liquid droplets takes place primarily in the first fiber bed. By providing a second fiber bed of relatively coarse fibers in intimate fiber to fiber contact with the first bed, re-entrainment is substantially prevented and drainage of collected liquid is effected. In accordance with the disclosure of the Argo et al. patent, the first bed is selected such that the residual saturation of this fiber bed against gas phase drag of the liquid phase is less than the residual saturation of the first bed against gravity drainage of the liquid phase. Conversely, the second bed is selected such that the residual saturation of the second fiber bed against gas phase drag of the liquid phase is greater than the residual saturation of the second fiber bed against gravity drainage of the liquid phase. Accordingly, the collected liquid is forced by the gas flow through the first bed to the interface between the beds and then drains by gravity either from the interface or within the second bed.
The Argo et al. patent describes both high efficiency mist eliminators, which typically operate at bed velocities up to about 100 feet per minute and can provide collection efficiencies up to about 99.9%, and high velocity separators, which can operate at bed velocities of 300 to 500 feet per minute and provide collection efficiencies of 85-95%. In a high efficiency mist eliminator as described by Argo et al., the first bed typically consists of fibers having a mean diameter of 5-20 microns and has a bed voidage of 85-95% while the second bed consists of fibers having a mean diameter of 25-35 microns and has a bed voidage of 85-95%. In a high velocity mist eliminator as described by Argo et al., the first bed consists of fibers having a mean diameter of 25-75 microns while the second bed consists of fibers having a mean diameter of 30 to 300 microns. Argo et al. state that the fibers of the respective beds can be constituted of various materials, glass fibers being particularly preferred.
However, it has been learned that, where the fiber diameter required for re-entrainment control is larger than about 100 microns, the use of glass fibers may not be practically feasible. Where fibers of such size are utilized, it is difficult to produce a glass fiber bed which has the requisite voidage and mechanical stability. One alternative to glass fibers is metal mesh. However, for use in removal of sulfuric acid mists, where volumetric flow rates are high so that mist eliminators preferably operate at high velocity, the use of metal mesh fiber beds has not heretofore been considered practicable because of the relatively rapid rates of corrosion suffered by most metals when exposed to sulfuric acid solutions. Fibers having a mean diameter in the range desirable for the second bed of a high velocity mist eliminator are generally too large for suitable fabrication from glass, but are small enough so that even modest corrosion rates may render a metal mesh bed inoperative within a fairly short period of time.
It is feasible, and has become standard practice, to repack fiber bed mist eliminators when the mist eliminator bed deteriorates or clogs with solids. However, in the case of a composite bed high velocity mist eliminator, in which the first fiber bed element consists of glass fibers which may survive a very extended exposure to even highly corrosive environments such as sulfuric acid, it is economically undesirable to repack the entire bed if only one component thereof, for example, a re-entrainment section comprising metal mesh, deteriorates. Where repacking of one or more elements must be done frequently, problems may also result from the time and space limitations within a particular installation. Shutdown, removal, and replacement of mist eliminator elements is time consuming and may cause excessive downtime.